As having high tensile strength, high stiffness and high heat resistance, glass fiber-reinforced resins are of great use as various materials. In particular, injection-molded products of resins of the type are the most popular because of their good moldability. However, they are defective in that their specific gravity increases with the increase in the glass fiber content and that their strength is lowered since glass fibers are often cut during molding. In addition, as being anisotropic, they are further defective in that their warping deformation is great. In order to overcome these defects, proposed was a method of using glass fiber-reinforced resin pellets in which the reinforcing glass fibers have the same length as each pellet to be from 2 to 100 mm and are oriented in parallel with each other (see, for example, JP-B Sho-63-37694, JP-A Hei-3-188131). Also proposed was a method of molding fiber-reinforced thermoplastic resin pellets with fibers having a length of from 10 to 100 mm and oriented in parallel with each other, to produce fiber-reinforced thermoplastic resin moldings with fibers entangled and having a length of from 5 to 100 mm (see JP-A Hei-6-198753). However, these methods are still problematic in that the molded products are heavy-weight, depending on the glass fiber content.
On the other hand, to reduce the weight of glass fiber-reinforced resin moldings, known is a foaming injection molding method in which is used a foaming agent (see JP-A Hei-7-247679). However, this method is problematic in that it requires a large amount of a foaming agent, that it is not easy to attain a foaming magnification of from 2 to 5 times, and that the glass fiber content is naturally limited. Also known is an injection-press molding method comprising injecting a foaming agent-containing resin into the cavity of an opened split mold followed by closing the mold. However, this method is still problematic in that, if a large amount of a foaming agent is used in order to obtain foamed moldings having a high foaming magnification, failed moldings having silver marks of vapor paths in their surface are formed, and that most of the moldings produced have poor strength and stiffness. For these reasons, at present, the known methods could not be put into practical use.
Another method of forming fiber-reinforced resin moldings is known, which comprises plasticizing a mixture of reinforcing fibers and a thermoplastic resin followed by molding it under compression, and which is characterized in that the compression for the surface of the molding in which the reinforcing fibers are intended to be restored expanded is partly released thereby raising the fibers existing in the surface layer of the molding, and/or forming voids partly inside the molding to make the molding have a sound-absorbing region (see WO96/25280) However, this molding method is to produce moldings having a specific, raised surface condition, and could not produce moldings with smooth surface. In addition, as comprising the compression molding step, this method is problematic in that its producibility is low, that the raw material mixture could not be highly homogenized, and that, if the mixture is highly homogenized, the reinforcing glass fibers are much broken. For those reasons, the use of the moldings produced by this method is limited.